1. Field of the Invention
The present invention relates to a solid state imaging device and a solid state imaging element, more particularly to a technology of controlling spectral characteristics of light receiving sections therein.
2. Description of the Related Art
In a conventional solid state imaging element (image sensor) for a color camera (digital camera) using a single imaging element, photodiodes are formed within a silicon substrate, on which a wiring layer is formed. Color filters for spatial modulation are formed on the wiring layer, and are provided with microlenses thereon.
Typical known color filter arrays are Bayer array for a primary color filter, and difference sequential array for a complementary color filter. Here, the Bayer array is such an array that lines (rows) having alternating green (G) and red (R) areas and lines having alternating green (G) and blue (B) areas are alternately arranged in the column direction, and that the green (G) areas are not aligned in the column direction between two adjacent lines (so that when viewed as a whole, the green (G) areas form a checkered or mosaic pattern). On the other hand, the difference sequential array is such an array that lines (rows) having alternating magenta (MG) and green (G) areas and lines having alternating yellow (YE) and cyan (CY) areas are alternately arranged in the column direction.
The conventional solid state imaging element described above creates a problem in that as the cell size decreases, the thickness (height) of a structure on the photodiodes, i.e. on the silicon substrate, significantly influences the performance of the solid state imaging element. For example, if the cell size is about 2 μm (that is photodiode size of about 1 μm2 with the aperture ratio being assumed as about 25%), a solid state imaging element of the CCD (Charge Coupled Device) type has a height (thickness) of about 2 μm to 3 μm from the surface of the silicon substrate to the upper surface of color filters, while a solid state image element of the CMOS (Complementary Metal Oxide Semiconductor) type has such height of about 5 μm to 6 μm. This causes that even with the aperture ration of about 25%, only a portion of incident light arrives at the photodiodes, depending on the angle of principal ray passing the microlenses. The reduction of the incident light arrival is particularly significant in the case of the solid state imaging element of the CMOS type, because it requires a thicker wiring layer than in the case of the CCD type.
As a solution to such problem, it may be considered to design a CCD type solid state imaging element so as to move and adapt the microlenses on the color filters to the angle of principal ray of the incident light. However, since such angle varies depending on the lens and zoom, this solution is limiting, and it is hard to mention this solution as being for general use. Besides, since the process of forming color filters is different from the semiconductor manufacturing process to form e.g. photodiodes, it is required to provide, for the color filters, a separate clean room and separate equipment therein such as stepper, coating equipment and cleaning equipment. This is another problem.
To solve these two problems, it is proposed to design a structure of a CMOS type solid state imaging element without using color filters as disclosed, for example, in the article by Richard F. Lyon and Paul M. Hubel, “Eyeing the Camera: into the Next Century”, IS&T/SID Tenth Color Imaging Conference: Color Science and Engineering Systems, Technologies, Applications; Scottsdale, Ariz.; Nov. 12, 2002; p. 349-355. According to this structure, photodiodes are stacked in three layers in the depth direction in a silicon substrate, whereby the respective photodiodes in a single cell extract wavelength components corresponding to the respective depths thereof in the stack. In other words, the photodiodes themselves are designed to have spectral characteristics. More specifically, a blue (B) component, i.e. short wavelength component, of incident light is detected and obtained from the shallowest photodiode, i.e. positioned closest to the substrate surface, and a red (R) component, long wavelength component, of incident light is detected and obtained from the deepest photodiode positioned farthest from the substrate surface, while a green (G) component, middle wavelength component, of incident light is detected and obtained from the photodiode positioned in the middle.
However, because of the three-layer stack, this structure is complex. In addition, since it is required to provide, in a single cell, the structure for obtaining outputs from the three photodiodes, the cell necessarily becomes large in size. In other words, it is difficult to reduce the cell size. Furthermore, since the photodiodes are stacked, it is not possible to set spectral characteristics for each photodiode individually.
It should be noted that it is understood that the purpose of the three-layer stack of photodiodes according to the above cited article is not to eliminate color filters, but to avoid spatial modulation of signals due to the color filters in the conventional structure with the two-dimensionally arranged photodiodes.